High power rf generation has typically required the serial combination of a master oscillator and power amplifier (MOPA), since; oscillators in general are not very efficient and are difficult to modulate at high power levels. In the microwave region, MOPA generation techniques involve conventional oscillators and amplifiers having electron guns that either operate in a continuous-wave (CW) regime or in pulses that are typically microseconds long. These are often called common beam modulation oscillators. The CW long-pulse electron beam employed by a common beam oscillator is accelerated by high voltage and then modulated at the oscillation frequency in a region of an electromagnetic field, e.g., within a resonator, that varies sinusoidally with time. MOPA rf generation is disadvantageous because the devices are generally complex and cumbersome.
An alternative to MOPA generation is embodied in a self-contained velocity modulation feedback oscillator such as the Klystron. The typical Klystron oscillator includes a thermionic cathode that produces a continuous flux of electrons from the cathode surface. The continuous beam of electrons from the cathode enters a cavity resonator called the input cavity in which the beam energy is modulated by the cavity's electromagnetic field. The modulated beam enters a field-free region and is allowed to "drift" until the slow electrons at the front of the beam are met by the fast electrons from the rear of the beam to form a "bunch" of electrons. At the proper location in space and time, the bunch of electrons enters a second electromagnetic field present in an output cavity in such a way as to give up energy to the electromagnetic field. Some of the energy from the output cavity electromagnetic field is fed back to the electromagnetic field in the input cavity in proper phase relationship to sustain oscillations.
The simple Klystron embodiment is relatively inefficient, in part, because many of the electrons initially emitted by the cathode are ineffectively modulated, and arrive either too soon or too late to give up energy to the electromagnetic field in the output cavity. These electrons are either simply lost or, in the worst case, extract energy from the electromagnetic field rather than adding energy to it. There are also limitations on the electron current that can be emitted from a thermionic cathode, with cathode life limited by electron depletion. The maximum temperature is limited by irreversible damage to the cathode. These temperature constraints necessitate relatively high accelerating voltages which, in turn, require the device to have x-ray shielding when producing a sustained power level.
Another device that has more recently been used to generate rf energy from an electron beam is the Lasertron. In the Lasertron, the thermionic cathode and the input cavity resonator of the Klystron are replaced by a photoelectric cathode that is activated ("gated") by a laser pulse to excite a pulsed beam of electrons from the cathode. The pulsed beam passes through a cavity resonator at the appropriate time and space relationship to add energy to the electromagnetic field present in the cavity resonator. By proper shaping of the gated pulse, the Lasertron achieves higher efficiency than the Klystron. A disadvantage of the Lasertron is that the laser-activated photoelectric cathodes used have a short lifetime. The Lasertron also suffers from the disadvantage that the number of electrons in the pulsed electron beam are directly related to the energy in the laser pulse, so that high rf power output demands powerful lasers, which are expensive and have a relatively short lifetime.